A number of users have reported this same issue and I have just figured out why it is happening. It turns out that in all versions of mIRC prior to v6.34 there was a mistake in the popups.ini file where item numbers for some lines overlapped.

mIRC v7.x is more strict about the formatting in INI files, so when a user upgrades to v7.x and keeps their old (pre-v6.34) popups.ini, the issue becomes apparent.

I will see if I can make the next version of mIRC check for the badly formatted popups.ini in order to cater for it.